A coating layer which encloses a golf ball main body is required to satisfy the following conditions: 1) the coating layer has high impact resistance (i.e., peeling resistance) so that it remains adhered onto the golf ball main body (i.e., the part of the golf ball other than the coating layer) when the golf ball is hit and deforms; and 2) the coating layer has high resistance to scuffing so that it is hard to get scuffed or cracked even when the golf ball is hit with a metal head of a short iron and receives high impact and friction. In general, when a golf ball is hit, it can deform 30% or more in diameter. Therefore, if the coating layer has poor adhesion to the golf ball main body, or has excessively high hardness with poor elasticity, the coating layer does not remain adhered onto the deformed surface of a golf ball main body, resulting in peeling off. On the other hand, if the coating layer is too soft, a problem arises that the coating layer easily gets scuffed or cracked. In addition, the coating layer is required to satisfy the aforementioned requirements not only in a dry condition but also in a wet condition, as a golf ball frequently gets wet during a play on a rainy day. When a golf ball gets wet, foams are generated in the coating layer or in an interface between the coating layer and the golf ball main body. The foams cause to decrease the hardness of the coating layer, or deteriorate the adhesion property between the coating layer and the golf ball main body.
In order to form a coating layer excellent in resistance to impact and scuffing, there has been conventionally developed a urethane-based coating material comprising a polyol as the chief material of the coating material and a polyisocyanate as the curing agent of the coating material. The urethane-based coating material is applied to the surface of a golf ball main body, and then is baked to promote a curing reaction of the polyol with the polyisocyanate.
In general, a golf ball main body has a cover made from an ionomer resin. When an ionomer resin is heated to high temperatures, the ionic bond of the ionomer resin is weakened. Therefore, if the coating material applied to an ionomer resin covered golf ball main body is dried at high temperatures, the elasticity and the hardness of the cover made from the ionomer resin are deteriorated. In that case, a drying process at lower temperature can avoid this deterioation. However, the curing reaction proceeds slowly, and therefore it takes a long time to dry the coating material, resulting in low productivity. For example, Japanese Unexamined Patent Publication No. 8-182775 discloses an urethane-based coating material produced by mixing a polyol and a polyisocyanate in such a manner that the amount of hydroxyl groups of polyol is excessive with respect to the amount of the isocyanate groups of polyisocyanate by molar ratio.
In this publication, in an attempt to improve the adhesion property between the coating layer and the cover of the golf ball, the polyol and polyisocyanate are mixed with each other. However, the urethane-based coating material requires a long drying time, for instance, 24 hours or longer, at 40 to 50.degree. C. This significantly lowers the productivity. In view thereof, it is desirable to produce an urethane-based coating material, which can be dried in a short time at a sufficiently high temperature to avoid the deterioration of the main body of the golf ball.
Recently, there has been developed an urethane-based coating material comprising a urethane polyol as the chief material. Urethane polyol belongs to a class of polymer polyol, because urethane polyol has urethane bonds in its main chain and has hydroxyl groups as end-groups. For example, Japanese Unexamined Patent Publication No. 9-59566 discloses a coating material for a golf ball including a urethane polyol. The urethane polyol has a weight-average molecular weight (hereinafter referred to as "Mw") of 10000 to 50000. However, this coating material adheres poorly to the surface of a golf ball main body. This is because the coating material has a smaller amount of free hydroxyl groups and free isocyanate groups than the ordinary urethane-based coating material, and therefore the coating material does not sufficiently react with the surface of the golf ball main body. The problem of poor adhesion becomes significant when the coating material is applied to a one-piece type golf ball main body which has a rubber core made of conventional rubber but no ionomer cover. Because the conventional rubber is less reactive than ionomer with a coating material. In the case of a one-piece type golf ball, a drying process at high temperatures also cannot be employed because the physical properties of the rubber core will likely deteriorate.